Over a million and a half miles of pipeline are buried in the United States alone. Such pipelines are used to transport hazardous gases and liquids, many at high pressures on the order of 1000 psi. The majority of these pipelines involve natural gas transport. There are approximately 1000 pipeline failures reported each year with a few involving loss of life or significant property loss.
Corrosion of the pipe material is the main cause of pipeline failure. Corrosion is an electrochemical process involving metal oxidation and mass and charge transport between an electrode and a surrounding electrolyte. The charge transport implies that an electrical current flows between locations on the pipe and from the pipe to external electrodes. A metallic pipeline can be an electrode and the soil an electrolyte so that the pipeline buried in soil forms the elements of an electrolytic cell. Some corrosion arises from the naturally occurring processes at specific locations on the pipe involving electrical current flow into the ambient soil electrolyte via the corrosion reaction Corrosion is often additionally caused or accelerated by voltages applied to a local region of the pipe by man-made structures, including local transit systems, power distribution systems and other terrestrial sources of stray voltages and currents.
As a result, early detection and control of corrosion are necessary to maintain the integrity of a pipeline. To accomplish this, pipelines are periodically tested or continuously monitored for indications of corrosion activity and where necessary the electrochemical environment of the pipeline is modified by established control techniques.
Corrosion monitoring has been conventionally accomplished by conducting pipe to soil potential surveys to determine whether the potential difference between pipe and soil exceeds a specified threshold potential of 850 millivolts relative to the copper-copper sulfate couple and generally defined as adequate to prevent corrosion. Typically the pipe to soil potential measurements are accomplished using a copper/copper sulfate electrode half cell, wherein one electrode is connected to the pipe and the other is in contact with the earth above the pipe. Measurements of this potential are made at intervals from several feet to fifty feet or more. Monitoring of on-pipe currents is typically accomplished with the use of two electrodes physically bonded to the pipe at selected locations with a typical spacing of 200 feet.
These currently available methods suffer limitations. First, the pipe to soil corrosion potential method does not determine the corrosion rate. Rather, it indicates a condition where corrosion could take place electrochemically. In practice, pipelines exhibiting a corrosion potential greater than the -0.85 V.sub.-- /CuSo.sub.4 limit cited previously have been found to be at risk of having unacceptable corrosion. Conversely, however, corrosion is a current phenomenon and the corrosion rate is proportional to the density of current between pipe and soil. There is a direct proportional relationship between the charge transfer processes responsible for the current and the metal mass transferred from the pipe electrode and the corrosion rate. In conventional electrochemical practice and in the present invention the central measured parameter is the ratio of the current density to the interfacial voltage between pipe and electrolyte viz the interfacial admittance or the inverse polarization resistance conventionally denoted as R.sub.p.
Additionally, another limitation is that the measured pipe to soil potential is an average over a relatively large length of pipe because of the need for direct connection to the pipe to effect these measurements and the soil conditions from pipe to the earth surface. As a result, small regions of high corrosion rate which could lead to pipe failure may not be found.
Furthermore, the use of IR drop electrodes, which are physically bonded to the pipe at selected locations, while providing an effective measurement of pipe current, is expensive and is limited because of the expense and the need to provide specific monitoring sites. In particular, in present practice the two electrodes bonded to the pipe which comprise an IR drop pair, are spaced 200 feet apart. This means that only currents which flow continuously over this distance are monitored. In areas where many pipes are present or where current loss from the pipe is suspected, the spatial resolution of this technique is inadequate.
The industry has applied negative voltages between pipe and soil (cathodic protection) to achieve protection as illustrated in FIG. 1 However, there is considerable uncertainty as to the actual value of the potential at specific pipe locations and as to what voltage level provides adequate protection for the pipe.
There are several monitoring techniques known from conventional electrochemistry which provide information which is directly related both to the presence of active corrosion and to corrosion rate monitoring. Since corrosion is a process involving both mass and charge transfer between a corroding solid and its environment, all of these methods involve measurement of the exchange current and more particularly the interfacial impedance given by the ratio of the current to the interfacial voltage.
There has been a report of the use of a magnetic field detector to observe corrosion currents in a small electrochemical cell in a laboratory environment. However, the authors did not monitor corrosion rates, determine interfacial impedances or suggest that corrosion rates could be determined by magnetic field detection. This report consisted of an article entitled "Detection of Magnetic Fields Generated by Electro-Chemical Corrosion" in the August 1986 issue of the Journal of the Electrochemical Society. Detection consisted of observing temporal changes in naturally occurring corrosion currents flowing two dissimilar metals in contact with an aqueous electrolyte. There was no use of an impressed voltage across the cell such as would have been required to conduct classical electrochemical corrosion monitoring experiments. The lack of this impressed voltage also means that correlation based signal processing techniques cannot be employed. In our work these signal processing techniques are used to separate corrosion induced magnetic signals in practical pipeline applications from other magnetic signals associated with geomagnetic noise, magnetic materials and structures in the earth and environmental noise associated with automobiles and other moving objects.
Magnetic detection has also been used for many years for monitoring the current distribution on buried pipelines. An article entitled "Electromagnetic Techniques for Monitoring Pipeline Coatings", presented as paper 311 at a corrosion conference in March 1987, is a recent example of this type of application. In this paper N. Frost suggested that an AC potential can be impressed between a pipeline and an electrode buried in the soil and spaced from the pipeline. The current distribution of the current along the pipe is detected via a magnetic sensor which monitors the AC magnetic field produced by the AC current on the pipe. He illustrates the positioning of an inductive (coil) type magnetic sensor to detect on-pipe current and especially the decrement in on-pipe current which occurs as current leaks into the soil through the coating on the pipe. Using the change in the gradient of the on-pipe current with position above the pipe, he shows that coating breaks can be detected. What he has not shown is neither the detection of the transverse current leaving the pipe using magnetic sensing means nor the use of either the on-pipe or transverse current to determine the electrochemical impedance of the interface at the pipe where corrosion is taking place. The impedance measurement is essential to the application of magnetic sensing for corrosion detection and corrosion rate monitoring. Moreover, it is required for the quantitative determination of corrosion activity on the buried pipeline.
The Frost system is based upon the analysis that electrical current leaks preferentially from breaks or holidays in the protective pipeline coating because these exhibit substantially reduced electrical resistance. While a completely protected pipeline would exhibit a substantially constant current gradient along its length as a result of a relatively uniformly distributed current leakage, a holiday can be detected by a substantial change in the current gradient plotted as a function of distance along the pipe.
One problem with a system which looks only at on-pipe current is its low sensitivity. That problem arises because the differential changes in on- pipe current are relatively small compared to the total on-pipe current. Thus, the environmental noise, such as changes in the earth's magnetic field, stray currents, and cathodic protection currents, makes it difficult to utilize such a system.
The prior art has also suggested a variety of electrochemical measurement techniques for determining the interfacial impedance associated with the pipe/soil interface. For example, there is the Tafel extrapolation method in which the interface is perturbed with a DC voltage. The linear polarization method uses a small ramp function potential applied to the interface. The small amplitude cyclic voltammetry method uses a sawtooth which is a repeated ramp function. Others have suggested impulse measurements and the use of harmonic signal analysis.
An example of the latter system is "Electro-Chemical Impedance Spectroscopy". Electro-Chemical Impedance Spectroscopy, abbreviated EIS is a linear AC impedance method which is a conventional electrochemical technique for measuring the chemical condition at an electrode/electrolyte interface. Its application to measuring the corrosion rate at pipe/soil interface was suggested in a published report entitled "Effectiveness of Cathodic Protection" by Thompson, Ruck, Walcott and Koch and published in 1987. In this method a small amplitude, AC potential is applied by a source between a direct connection to the pipe and an electrode buried in the soil and spaced from the pipe. The amplitude and phase of the resulting source current with respect to the applied source voltage is detected for each of a plurality of source signal frequencies.
The total electrical current passing through the pipeline, soil, and electrode is assumed to be reasonably controlled by Randle's equivalent circuit or other equivalent circuit, several of which have been developed by the prior art workers. For example, equivalent circuits are discussed in an article in Corrosion Science entitled "Utilization Of The Specific Pseudo-Capacitance For Determination Of The Area Of Corroding Steel Surfaces" published in the August 1988 edition, volume 44, No. 8 and in a further article in the same issue entitled "Equivalent Circuits Representing The Impedance Of A Corroding Surface".
However, to adequately represent the current loss distribution on a large extended structure such as a pipeline, an equivalent circuit model such as that shown in FIG. 10 is required. At each location along the pipe there is an impedance value describing local current flow and hence local corrosion activity.
In EIS, the amplitude and phase data for the current at each frequency and applied potential are used to calculate a complex impedance for each frequency, including both the amplitude and phase of that impedance. This measured impedance of the circuit to which the AC source is applied for each of several frequencies may then be equated with the algebraic expression for the impedance of the equivalent circuit and these simultaneous equations are solved for values of the circuit elements. As is described in the prior art, the circuit elements, and particularly the interfacial resistance and capacitance along the pipe to soil interface provide indications of both interface condition, that is the presence or absence of holidays and corrosion, and also the corrosion rate. Often the process of impedance calculation is carried out under computer control.
A principal problem, however, with the EIS system and other prior art electrochemical systems described above is that they require physical contact with the pipe. Operationally, this is a major limitation when the pipe is under pavement or otherwise inaccessible. In addition, conventional measurements are applicable only to total currents passing through a relatively long spa of pipe. Thus, the measurements are essentially an average over a long span of pipe and do not provide information about the local condition of the pipe.
Thus, the disadvantages of conventional electrochemical methods for pipe corrosion detection are that they determine an average corrosion rate over an entire pipe length and therefore obscure small regions of high corrosion activity; they are not directly applicable under conditions of cathodic protection; and errors result from soil resistivity effects.
There is therefore a need for an improved system overcoming the above mentioned disadvantages of the current technology and providing for a non-contact system which can measure local current distribution and local impedance values.